Ground Penetrating Radar Method & IDS RIS Structure GPR Systems

Description Ground penetrating radar (commonly called GPR) is a geophysical method that has been developed for shallow, high-resolution, subsurface/structure investigations. GPR uses high frequency electromagnetic waves (generally 25 MHz to 2.0GHz) to acquire subsurface information. Energy is propagated downward into the ground and is partially reflected back to the surface from boundaries at which there are electrical property contrasts (Figure 1. Shows the GPR schematic process). GPR is a method that is commonly used for civil engineering-geotechnical, geological, environmental, archaeological, and other shallow investigations (1-40m).

Ground Penetrating Radar Method & IDS RIS Structure GPR Systems

Data Storage

Control Unit

Data Display

TransmitterAntenna

ReceiverAntenna

Ground Surface

Soil

Direct Arrival

Bedrock

ea

Ey

Rf

cted

ner

g

r

Scattred Energy

Buried ObjectRefrected Energy

mt

ru

s

Transi te

Pl

e

E

Rf a

cted

ner

g

er

y

Figure 1. GPR Principle of Operation (from ASTM 6432).

Typical Uses GPR is used to locate and map objects such as pipes, drums, tanks, cables, and underground features. Other applications include structure (concrete) and pavements (highways, trains and airports) analysis, mapping geologic conditions that include depth to bedrock, depth to the water table, depth and thickness of soil and sediment strata on land and under fresh water bodies, and the location of subsurface cavities, sinkholes and fractures in bedrock, mapping landfill and trench boundaries, mapping contaminants, and conducting archaeological investigations.

Performance Specs The GPR method is site specific in its performance depending upon the surface and subsurface conditions. Performance specifications include requirements for or information about reflections, depth of investigation, resolution, interferences, quality control, and precision and bias. Reflections Reflections are created by an abrupt change in the electrical and magnetic properties of the material the electromagnetic waves are traveling through. In most situations, magnetic effects are small. Most GPR reflections are due to changes in the relative permittivity of material. The greater the change in properties the more signal is reflected. In addition to having a sufficient electromagnetic property contrast, the boundary between the two materials needs to be sharp. This is particularly evident (and favourable to GPR) in structure studies where concrete and asphalt have very distinct dielectric constant and conductivity of usual embedding feature.

Depth of Penetration The principle limiting factor in depth of penetration of the GPR method is attenuation of the electromagnetic wave in the earth materials. The attenuation predominantly results from the conversion of electromagnetic energy to thermal energy due to high conductivities of the soil, rock, and fluids. Scattering of electromagnetic energy may become a dominant factor in attenuation if a large number of inhomogeneties exist on a scale equal to the wavelength of the radar wave. GPR depth of penetration can be more than 30 meters in materials having a conductivity of a few milliSiemens/meter. However, penetration is commonly less than 10 meters in most soil and rock. Penetration in mineralogic clays and in materials having conductive pore fluids may be limited to less than 1 meter. For structural studies where penetration is a relative issue, material composition provides favourable conditions to the application of hi resolution antennas that can easily scan the desired shallow depth interval.

Interferences The GPR method is sensitive to unwanted signals (noise) caused by various geologic and cultural factors. Geologic (natural) sources of noise can be caused by boulders, animal burrows, tree roots, and other inhomogeneties can cause unwanted reflections or scattering. Cultural sources of noise can include reflections from nearby vehicles, buildings, fences, power lines, and trees. Electromagnetic transmissions from cellular telephones, two-way radios, television, and radio and microwave transmitters may cause noise on GPR records. Shielded antennas are used to limit these types of reflections.

Resolution GPR provides the highest lateral and vertical resolution of any surface geophysical method. Various frequency antennas (25 to 2000 MHz) can be selected so that the resulting data can be optimized to the projects needs. Lower frequency provides greater penetration with less resolution. Higher frequencies provide less penetration with higher resolution. Resolution of a few centimetres and less can be obtained with high frequency antennas (2 GHz) at shallow depths, while lower frequency antennas (25 MHz) may have a resolution of approximately one meter at greater depths. Horizontal resolution is determined by the distance between station measurements, and/or the sample rate / pulse repetition frequency, the towing speed of the antenna, and the frequency of the antenna.

Performance Specs The GPR method is site specific in its performance depending upon the surface and subsurface conditions. Performance specifications include requirements for or information about reflections, depth of investigation, resolution, interferences, quality control, and precision and bias. Reflections Reflections are created by an abrupt change in the electrical and magnetic properties of the material the electromagnetic waves are traveling through. In most situations, magnetic effects are small. Most GPR reflections are due to changes in the relative permittivity of material. The greater the change in properties the more signal is reflected. In addition to having a sufficient electromagnetic property contrast, the boundary between the two materials needs to be sharp. This is particularly evident (and favourable to GPR) in structure studies where concrete and asphalt have very distinct dielectric constant and conductivity of usual embedding feature.

Depth of Penetration The principle limiting factor in depth of penetration of the GPR method is attenuation of the electromagnetic wave in the earth materials. The attenuation predominantly results from the conversion of electromagnetic energy to thermal energy due to high conductivities of the soil, rock, and fluids. Scattering of electromagnetic energy may become a dominant factor in attenuation if a large number of inhomogeneties exist on a scale equal to the wavelength of the radar wave. GPR depth of penetration can be more than 30 meters in materials having a conductivity of a few milliSiemens/meter. However, penetration is commonly less than 10 meters in most soil and rock. Penetration in mineralogic clays and in materials having conductive pore fluids may be limited to less than 1 meter. For structural studies where penetration is a relative issue, material composition provides favourable conditions to the application of hi resolution antennas that can easily scan the desired shallow depth interval.

Interferences The GPR method is sensitive to unwanted signals (noise) caused by various geologic and cultural factors. Geologic (natural) sources of noise can be caused by boulders, animal burrows, tree roots, and other inhomogeneties can cause unwanted reflections or scattering. Cultural sources of noise can include reflections from nearby vehicles, buildings, fences, power lines, and trees. Electromagnetic transmissions from cellular telephones, two-way radios, television, and radio and microwave transmitters may cause noise on GPR records. Shielded antennas are used to limit these types of reflections.

Resolution GPR provides the highest lateral and vertical resolution of any surface geophysical method. Various frequency antennas (25 to 2000 MHz) can be selected so that the resulting data can be optimized to the projects needs. Lower frequency provides greater penetration with less resolution. Higher frequencies provide less penetration with higher resolution. Resolution of a few centimetres and less can be obtained with high frequency antennas (2 GHz) at shallow depths, while lower frequency antennas (25 MHz) may have a resolution of approximately one meter at greater depths. Horizontal resolution is determined by the distance between station measurements, and/or the sample rate / pulse repetition frequency, the towing speed of the antenna, and the frequency of the antenna.

Precision and Bias Precision is a measure of the repeatability between measurements. Precision can be affected by the location of the antennas, the tow speed, the coupling of the antennas to the ground surface, the variations in soil conditions, and the ability and care involved in picking reflections. Assuming that soil conditions remain the same (that is, soil moisture), repeatability of radar measurements can be 100%. Bias is defined as a measure of closeness to the truth. The accuracy of a GPR survey is dependent upon picking travel times, processing and interpretation, and site-specific limitations, such as unknown changes in radar velocities (lateral and vertical) or the presence of steeply dipping layers. In structure investigations the relative homogeneous behaviour of materials like concrete and asphalt reduces the bias and increase the precision of the GPR application.

GPR measurements are relatively easy to make and are not intrusive. Antennas may be pulled by hand or with a vehicle from 0.8 to 8 kph, or more, that can produce considerable data/unit time. GPR data can often be interpreted right in the field without data processing. Graphic displays of GPR data often resemble geologic cross sections. When GPR data are collected on closely spaced (less than 1 meter for geology or 2 to 5 cm for hi resolution structural studies) lines, these data can be used to generate three dimensional views of radar data greatly improving the ability to interpret subsurface conditions.

The major limitation of GPR is its site specific performance. Often, the depth of penetration is limited by the presence of mineralogic clays or high conductivity pore fluid. For concrete, structure and pavement applications GPR generally is less affected from the above mentioned limitations. The main concern is due to the amount of rebars that, in some piles, may limit (due to their proximity and density) the transmission of the signal.

the Ground Penetrating Radar (GPR) method can be performed on concrete elements, standard framed or masonry walls, concrete and asphalt pavements, and soil.

The method is primarily applied to: locate and measure the depth of steel reinforcement, post-tensioning and prestress tendons or ducts, locate and measure the depth of embedded metallic or plastic conduits in concrete slabs, walls, or structural members, identify defect or damages inside concrete (voids, fractures, delaminations,), define vertical and lateral variations in concrete composition, identify humidity, water penetration or dissolution areas inside and behind concrete slabs, characterize subsidence or sink hole behind concrete structures/slabs, define areas of corrosion in reinforced bridge decks or other elements, determine thicknesses of members with little or no reinforcement, measure pavement thickness and properties, and locate subgrade voids below concrete slabs or behind retaining walls. Borehole radar has been successfully applied in the definition the top of the pile was not free) and in horizontal drillholes the existence of fractures and their structural patterns.

The fields of interest for these applications are: Installation engineering audit for concrete structure and buildings. Important factor both for newly constructed and old structures needing a new seismic recalification and remodeling work.Foundation studies. Integrity studies of : Dikes, concrete channels, walls, dams, sewers, tunnels, piers etc. Rigid and flexible pavement studies. Metro and railways pavement and structure studies.

GPR Advantages

GPR Limitations

Regarding Structures and Concrete,

,

,

,,,,,

,

,

,

,,,,

For NDT concrete and structure applications, the selection of antenna is dependent on the desired feature resolution and depth of penetration and the typical range is 600 to 2.0 GHz. In this sector IDS is specialized to offer multichannel/multifrequancy units configurable in arrays to provide improved resolution and investigation capabilities. Ground penetrating radar is now a widely accepted field screening technology for characterizing and imaging subsurface conditions.

The American Concrete Institute (ACI) presents the GPR method in its Reported by ACI Committee

228. Additional norms are presented in the reference list.

The most common mode of GPR data acquisition is referred to as the reflection profiling method. In the reflection mode of operation, a radar wave is transmitted, received and recorded each time the antenna has been moved a fixed distance across the surface of the ground, in a borehole, or across any other material that is being investigated. Three-dimensional ground-penetrating radar (GPR) consists of collecting GPR data on closely spaced (less than 1 meter) lines using an array of antennas of the same frequency or different frequencies. Powerful computers are then used to composite these lines into a three-dimensional data volume that can be observed from any angle using any subset of the data. Transillumination measurements can be used in locations such as walls, structures, and boreholes where the transmitter and receiver can be put on opposite sides of a medium so as to look through it. Tomographic reconstruction techniques can be used to image the volume between the measurement points.

IDS Antennas The available single transducers are: ! IDSTR600, 600 MHz antenna ! IDSTR900, 900 MHz antenna ! IDSTR1600, 1600 MHz antenna ! Newly developed IDSTR2000, 2000 MHz antenna ! Newly developed IDSTR2000BP, 2000 MHz bipolar antenna

These antennas can be used in a monostatic configuration or combined in arrays. To the already existing Hiress 4x1.6 GHz antenna array or the 600-1600 Mhz dual antenna configuration, the user can easily ensemble its own kit of multichannel acquisition. The 600-1600 MHz array available also in a single box is effectively used also in pavement studies.

The American Society for Testing and Materials (ASTM) has an approved Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation (ASTM 6432-99). Nondestructive Test Methods for Evaluation of Concrete in Structures, norm ACI 228.2R-98

Mode of Operation

! High frequency for improved performance (highest frequency on the market). ! The bi polar approach allows joint acquisition of longitudinal and transversal lines to permit

enhanced detection capabilities and survey time reduction. ! Rugged, lightweight and highly maneuverable. ! Optical reader and standard referencing. Dedicated carpet grid for easy referencing. ! Fully integrated with standard IDS 2D and 3D software. A special module (Gred S 3D) provides

specific engineering applications solutions. ! Wide variety of accessories for versatile use both in vertical and horizontal configurations, for contact

hand held, kart mounted or air launched applications.

IDS software processing Three dimensional displays are fundamentally block views of GPR traces that are recorded at different positions on the surface. Data are usually recorded along profile lines where accurate location of each trace is critical to producing accurate 3D displays. Normally, 3D block views are constructed, then they may be viewed in a variety of ways, including as a solid block or as block slices. Obtaining good three-dimensional images are very useful for interpreting specific targets eand especially in structure applications. Targets of interest are generally easier to identify and isolate on three dimensional data sets than on conventional two dimensional profile lines.

The IDS GRED 3D software, designed for GPR structure data processing and interpretation presents the following features: ! Easy import and processing of multichannelmultifrequency GPR data ! GPS location and topography support ! Data quality control module ! Flexible data handling and visualization - 3D cube rendering and slicing. ! Spatial data representation and processing ! 2D and 3D picking ! Enhanced interpretation tools ! Standard printing and image export ! SEG Y import and export ! Spatial data export ! Dedicated engineering modules with advanced interpretation tools

References ! ASTM D6432-99. Standard Guide for Using the Surface Ground Penetrating Radar Method for

Subsurface Investigation ! Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface

Investigation ! AASHTO PP 40, "Application of Ground Penetrating Radar (GPR) to Highways". ! AASHTO TP-36, "Standard Test Method for Evaluating Asphalt-Covered Concrete Bridge Decks

Using ! Pulsed Radar". ! ACI 228.2R, "Nondestructive Test Methods for Evaluation of Concrete in Structures", ACI

Manual of Concrete Practice, Part2, Construction Practices and Inspection, Pavements, ACI International.

! ASTM D4748-98, "Standard Test Method for Determining the Thickness of Bound Pavement Layers Using Short-Pulse Radar", Book of Standards Volume 04.03, ASTM International.

! ASTM D6087, "Standard Test Method for Evaluating Asphalt-Covered Concrete Bridge Decks Using GROUND Penetrating Radar", Book of Standards Volume 04.03, ASTM International.

! ASTM D6432, "Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation", Book of Standards Volume 04.09, ASTM International.

! FHWA-HIF-00-015, "Ground Penetrating Radar for Measuring Pavement Layer Thickness". ! NCHRP Synthesis No. 255, "Ground Penetrating Radar for Evaluating Subsurface Conditions for

Transportation Facilities", A Synthesis for Highway Practice, TRB Synthesis Studies. ! SHRP C-101, "Condition Evaluation of Concrete Bridges Relative to Reinforcement Corrosion",

Vols. 1-8. SHRP Reports S-323 through S-330. ! SHRP Catalogue No. 4008, "Pavement Thickness Software Using Radar".

APPLICATION EXAMPLES Inspection of a concrete wall The radar section shown in Fig. 1 was performed using the IDSTR1600 antenna (1600 MHz) on an internal wall of a building. The section shows: ! The presence of two distinct zones (zone A and zone B), characterised by a different penetration

of the electromagnetic signal (see Fig. 1): this difference is due to a variation in the thickness of the wall;

! The presence of a group of hyperbolas due to the presence of a reinforced pillar (see Fig. 1).

Fig. 1 Radar section performed on a wall (1200 MHz antenna)

Fig. 2 reconstructs the structure of the pillar on the basis of the other radar information gained from the opposite side of the wall.

Fig. 2 Diagram showing the position of the reinforced pillar detected in the radar section

Inspection of a bridge deck When inspecting a bridge deck with the GPR, it is possible to reconstruct the geometry of the metallic structure of the bridge. Fig. 3 and Fig. 4 show the bridge which has been surveyed with the RIS K2 system.

Fig. 3 - The investigated bridge: a view from above

Fig. 4 - The investigated bridge: a view from below

The radar investigation was performed on the entire top surface of the bridge and on accessible portions of the lower and side surfaces. The construction characteristics of the different parts observed are reported below and shown schematically in (see Fig. 5): a) Part 1 embankment with an asphalt surface b) Part 2 reinforced structure (beam) with a thickness of 0.80m, underneath a reinforced slab, thickness 0.50m c) Part 3 reinforced junction slab, thickness 0.50m d) Part 4 reinforced concrete beams with a trapezoidal section, maximum thickness 1.00m, supporting a reinforced slab, thickness 0.50m e) Part 5 - reinforced junction slab, thickness 0.50m f) Part 6 Embankment with an asphalt surface

Fig. 5 Plan and perspective view of the investigated flyover

Some transversal radar sections of part 2 are shown below (Fig. 6 and Fig. 7). These were performed with the 1600 MHz high frequency antenna. The presence of a dense, electrically welded mesh can be observed. The number of steel rods that make up this mesh, and their orientation can be obtained from distinct signals shown in the first radar section (Fig. 6). Analogous results were obtained from reading the corresponding transversal radar map, shown in Fig. 7, acquired using the 600 MHz antenna. Obviously, the resolution that can be obtained with this antenna is inferior to that of the previous antenna, but it enables a deeper penetration to be achieved, so that the lower edge of the slab can be identified.

Fig. 6 High frequency radar section (1200 Mhz)

Fig. 7 Medium frequency radar section (600 Mhz)

Inspection of a floor Some scans were performed on the floor of a building according to the diagram shown in Fig. 8.

Fig. 8 Plan of the investigated floor

The portion of radar section shown in Fig. 9 was performed in the direction L2 shown in Fig. 8. The section clearly shows a continuous series of radar echoes at a depth of around 10cm beneath the floor surface, and at a distance of 50cm from each other (see black circle). The radar section shown in Fig. 10 was performed perpendicularly to the section shown in Fig. 9 and relates to the scan line L1. Here, the presence of a series of hyperbolas can clearly be seen at a depth of around 10cm from the floor surface, at intervals of around 50cm. The same section also shows a second level of anomalies deeper down (at around 40cm below the floor surface) at intervals of around 1m.

Fig. 9 Radar section produced with the 1600 MHz antenna in the direction L2

Fig. 10 Radar section produced using the 1600 MHz antenna in the direction L1

Using the radar maps, it was possible to reconstruct the geometry of the joists and bricks that made up the floor, as shown in Fig. 11.

Fig. 11 Diagram showing the structural elements of the floor (beams and joists)

Inspection of a floor with the TR2000BP, 2 GHz bipolar antenna The newly developed 2 GHz bipolar antenna guarantees unmatched resolution characteristics and the possibility to simultaneously collect data in two polarizations, thus using polarization effects for having best results in detecting metallic and non metallic targets. The bipolar antenna uses four dipoles (two for transmitting, two for receiving), differently oriented; two (one for transmitting the other for receiving) are with their long axis parallel to the scan direction, whereas the other two lay perpendicularly to the scan direction. With respect to the picture above where a red arrow indicates the scan direction, metallic targets as rebars or cables are best seen by the dipoles parallel to them, non metallic objects like PVC pipes or cavities are better detected by the dipoles that lay perpendicularly to them. Another benefit of collecting data in both polarization is due to the fact that echoes generated by metallic targets perpendicular to the scan direction are weakly seen in data collected by dipoles parallel to the antenna route, so that other objects below them can be more easily detected (for instance the concrete slab). This phenomenon can be appreciated in the following pictures concerning a collection performed on a pavement. Finally, when using the TR2000BP antenna, there is no need of performing scans in X and Y directions (grid); all the data required for producing a 3-D imaging of the surveyed area, are collected in a single scan.

2.0 GHz Bi Polar Antenna. Jointly Acquired Longitudinal and Transversal Polarizations Scans

The Longitudinal Polarization scan uncovers the deep reinforced concrete structure and the hollow

The Transversal Polarization scan characterizes the iron net below the surface.

Inspection of a building damaged by an earthquake This case history reports on some measurements performed on a building damaged by an earthquake. Main purposes of the GPR survey were related to the identification of anomalies inside the walls produced by the injection of cement mortar performed to reinforce the structure after a previous earthquake. Since a high resolution was required, the radar investigations have been performed with high frequency (1600 MHz) and medium frequency (600MHz) radar antennas. In order to verify the significance of anomalies in the brickwork detected by the radar techniques, micro coring was later performed in some significant points. The micro coring samples were performed using a rotating drill equipped with a simple corer 30 cm long and 3.7cm diameter. The drilling was performed to a maximum depth of 50 cm.

The part of the building investigated occupied a surface area of about 7m2. The radar scans were performed at 50 cm intervals both in longitudinal and transverse directions. With respect to Fig. 12, the transverse scans were performed from the bottom upwards, whereas the longitudinal scans were performed from left to right. In total, 5 micro coring samples were performed on the facade of the building, S1, S2, S3, S4 and S5 (Fig. 12).

The radar analysis detected two zones in the wall which presented anomalies. They were defined as Zone 1 and Zone 2. The Fig. 13 compares the radar sections and the corresponding GPR signal penetration maps with the results of the sampling performed both within (S1e S2) and outside (S3 e S5) Zone 2.The radar data highlight the presence of two zones ("A" and "B") with distinctly different characteristics.

In part A the radar data show a texturally homogeneous facies and a reduced penetration of the signal; thanks to the micro coring sampling it was possible to verify the presence of conglomerates

of cement that had been injected during a previous reinforcing operation, which showed good homogeneous characteristics.

Inside part B the textural facies of the radar map is characterised by diffuse hyperbolic echoes and the segmentation map shows an increased penetration of the signal; this part proved to consist of limestone blocks (red and white scaglia), which give the structure non homogeneous and fragmented characteristics.

Fig. 12 position of the scans executed

Zona 2Zona 1

S1 S2

S5 S3 S4

Fig. 13 - Building n74. Comparison between: e.m. signal penetration maps, radar maps (1600MHz) and micro coring stratigraphies

(the traces of the radar maps are indicated by the red lines in Fig. 12)